1. Field of the Invention
The present invention relates to a hollow package, particularly a hollow package in which a package body having a recess is sealed with a transparent sealing plate, and also relates to a method for fabricating the same.
2. Description of the Related Art
A solid-state image apparatus of a typical leadless chip carrier (LCC) generally has a structure, as shown in FIG. 4, in which a ceramic package body having a recess with a step receives a solid-state image device, and the package body is sealed with a glass plate. In the drawing, symbol a represents a first layer of the ceramic package, which is flat, and a metallized lead d is formed thereon, extending from the side to the outer bottom. Symbol b represents a second layer of the ceramic package, which is in the shape of a rectangular frame, and a metallized lead d is formed thereon, extending from the upper surface to the side, and also connecting to the lead d on the first layer a. Symbol c represents a third layer of the ceramic package, which is in the shape of a rectangular frame, and an opening of the third layer c is larger than an opening of the second layer b. The two openings form a recess e, for receiving a solid-state image device, having a step with the first layer a serving as the inner bottom.
Symbol f represents a solid-state image device received in recess e on the ceramic package, symbol g represents bonding wires connecting between electrodes of the solid-state image device f and the metallized leads d, and symbol h represents a transparent sealing plate composed of glass bonded onto the upper surface of the package with an adhesive i therebetween for sealing the recess e.
Next, a method for fabricating the package will be described. Unsintered package sheets a, b, and c (corresponding to the first layer a, second layer b, and third layer c), which are so-called "green sheets" or "green ceramic sheets", are prepared. The green lead d is formed on the individual sheets a and b. The individual sheets a, b, and c are deposited at predetermined positions and are sintered, whereby, the package body is obtained. Next, the metallized lead d is formed by plating, and die bonding is performed to the solid-state image device f, followed by wire bonding. Lastly, the transparent sealing plate h composed of glass is bonded onto the upper surface of the package body. Thus, a solid-state image apparatus is complete, in which the solid-state image device is received in the ceramic hollow package.
Because of the high cost of materials and processing for ceramic hollow packages, it has been difficult to reduce the fabrication costs of solid-state image apparatuses. Therefore, the present inventor and others have made attempts to fabricate hollow packages from a resin instead of ceramic by applying the molded interconnect device (MID) technique widely used in various electronic devices to hollow packages. This is because an MID is an electronic component in which a circuit (three-dimensional circuit) is formed on a three-dimensional part produced by injection molding, and a resin material which is considerably less expensive in comparison with a ceramic material is used, and also the inexpensive injection molding technique which is easier and faster than sintering is used for formation. In particular, it is greatly advantageous that injection molding be performed once to enable the formation of a package body even in the case of a three-dimensional part, which is in contrast to the complex fabrication of a ceramic hollow package requiring the steps of forming three-layers of green sheets, depositing them at correct positions, and sintering.
The current MID technique, however, has been found to be unsuitable for hollow packages which receive semiconductor devices such as solid-state image devices. The reason is that in the general MID, a thermoplastic resin having a high thermal coefficient of linear expansion, for example, a liquid crystal polymer (LCP) or PPS, is used, and there is a large difference in thermal coefficient of linear expansion between the thermoplastic resin and glass used as the transparent sealing plate. Therefore, if the hollow package is sealed with the transparent sealing plate, either the sealing plate or the resin forming the package body will be easily cracked, resulting in difficulty in maintaining air-tightness.
Also, since leads, i.e., lands, and areas between leads, i.e., spaces, intervene between the transparent sealing plate and the package body which are to be bonded together, if the general adhesion technique using a thermosetting resin is employed, an internal pressure between the package body and the transparent sealing plate increases because of the heating for adhesion, resulting in air leakage through the spaces of the lands-and-spaces. This also does not allow air-tightness to be maintained.
In the case of a ceramic hollow package, since a plurality of sheets having a lead on the surface are deposited, a lead for connecting between the interior of the hollow package and the exterior can be formed even without placing wires on the upper surface of the hollow package. On the other hand, in the case of a package body formed by using the MID technique, unless a through-hole is formed on the package body, and the interior of the package body and the external lead are connected via the through-hole, the lead must be formed on the surface of the package body, and thus, the lead inevitably runs on the upper surface of the package body which is the connecting section between the package body and the transparent sealing plate. As a result, the lands-and-spaces are formed on the upper surface of the package. The thickness of the spaces is substantially equal to the thickness of the lead, for example, 20 to 30 .mu.m. Generally, since a thermosetting resin is used as the adhesive, if the conventional technique is followed, heat treatment must be performed to generate adhesive bonding in the adhesive material. If heat treatment is performed, the internal pressure within the hollow package increases inevitably and even if a resin for adhesion is buried in the spaces, air leakage occurs, and thus, it is impossible or extremely difficult to maintain air-tightness.